Multicast VLAN Registration (MVR) is an innovative feature that is key for metro service providers interested in delivering residential voice and video services. MVR involves the creation of separate dedicated VLANs constructed specifically for multicast traffic distribution. When deployed, each switch that receives a multicast stream from the multicast VLAN will examine each multicast group and will internally bridge the multicast VLAN traffic to the particular subscriber's VLAN.
Multicast VLAN Registration (MVR) is designed for applications using wide-scale deployment of multicast traffic across an Ethernet ring-based service provider network (for example, the broadcast of multiple television channels over a service-provider network). MVR allows a subscriber on a port to subscribe and unsubscribe to a multicast stream on the network-wide multicast VLAN.
FIG. 1 depicts a ring network topology that can utilize MVR and FIG. 2 depicts three access switches (A, B, and C) included in the topology of FIG. 1. Although the ring topology saves costs when constructing a network the trunk lines between the switches form a bandwidth bottleneck. Accordingly, techniques for efficiently utilizing available bandwidth are critical when using the ring topology.
An important requirement for the metro network is to ensure that one subscriber does not see the traffic generated by another subscriber or targeted for another subscriber via only layer 2 connectivity without any filtering based on layer 3 and beyond information. Administrators of the network may have different access control configuration for each user depending on the type of service subscribed. Communication between any two subscriber ports should only be possible via a device which can filter based on layer 3 and beyond information.
However, subscriber ports on the same layer 2 access switch can still communicate with each other through layer 2 only connectivity. In switches manufactured by the assignee of the present application the protected port feature isolates subscriber ports on the same switch. No unicast, multicast or broadcast traffic is forwarded between protected ports at layer 2, all communication between these ports is only possible through a layer 3 device.
A protected port does not forward any traffic (unicast, multicast or broadcast) to any other port that is also a protected port.
When a router receives a multicast packet it sends a copy of the packet to each connected VLAN having member of the multicast group. Because subscribers connected to different layer 2 access switches belong to different VLANs (a VLAN maps to a IP subnet), normal usage of IP multicast would result in duplication of subscribed channels for each VLAN. For example, if a first subscriber connected to layer 2 switch A wants to watch channel 1 and a second subscriber connected to layer 2 access switch B also wants to watch channel 1, two streams of channel 1 multicast would be sent through the GE VLAN trunk because each subscriber is on a different VLAN. One stream would be tagged with the VLAN assigned to the first subscriber's port and the other stream would be tagged with the VLAN assigned to the second subscriber's port. As an example, if each video stream were multicast at 5 Mbps duplicating the stream for each subscriber wishing to view the same would waste significant bandwidth.
MVR is designed to eliminate the need to duplicate multicast traffic for a TV channel for subscribers in each VLAN. Using MVR, multicast traffic for all channels is sent only once around the VLAN trunk, specifically only on the multicast VLAN. The layer 2 switches are configured to bridge multicast traffic from the multicast VLAN to the local VLAN defined on the layer 2 switch that includes an authorized subscriber.
Much of the specification for IP Multicasting is defined in the Internet Group Management Protocol (IGMP) in the Internet Task Force Request for Comments (IETF RFC) 1112 which is available on the sis.ohio-state.edu website with the extension /htbin/rfc/rfc112.html.
The Internet Group Management Protocol (IGMP) is used to dynamically register individual hosts in multicast group on a particular VLAN. IGMP v.2 utilizes membership query, membership report, and leave group messages.
Hosts send out a “join” which consists of sending IGMP membership reports corresponding to a particular multicast group to indicate that they are interested in joining that group. The layer 3 device periodically sends out an IGMP membership query to verify that at least one host on the subnet is still interested in receiving traffic directed to that group. When there is no reply to three consecutive IGMP membership queries, the router times out the group and stops forwarding traffic directed toward that group.
The hosts send out a “leave” by using the leave group message to actively communicate to the local multicast layer 3 device their intention to leave the group. The layer 3 device then sends out a group-specific query and determines whether there are any remaining hosts interested in receiving the traffic. If there are no replies, the layer 3 device times out the group and stops forwarding the traffic.
The default behavior for a Layer 2 switch is to forward all multicast traffic to every port that belongs to the destination LAN on the switch. This would defeat the purpose of the switch, which is to limit traffic to the ports that need to receive the data. IGMP snooping is used to efficiently deal with IP multicast in a layer 2 switching environment.
Standard IGMP snooping requires the subscriber switch to examine, or snoop, some Layer 3 information in the IGMP packets sent between the hosts and the router. When the switch hears the IGMP host report from a host for a particular multicast group, the switch adds the host's port number to the associated multicast table entry. When the switch hears the IGMP leave group message from a host, it removes the host's port from the table entry. Because IGMP control messages are transmitted as multicast packets, they are indistinguishable from multicast data at Layer 2. A switch running IGMP snooping examine every multicast data packet to check whether it contains any pertinent IGMP must control information.
In MVR the layer 2 access switches implement an enhanced technique of IGMP snooping to eliminate the need to duplicate multicast traffic for a video stream intended for subscribers in different VLANs. Instead, multicast traffic for all video streams is sent only once around the VLAN trunk, specifically only on the multicast VLAN
In order to achieve such functionality, the IGMP joins and leaves coming from a subscriber must look like they were initiated by a port in the multicast VLAN, rather than the VLAN the subscriber is assigned to. These IGMP joins and leaves dynamically register for streams of multicast traffic in the multicast VLAN on the layer 3 distribution layer device. The access layer switch modifies the hardware forwarding behavior to allow the traffic to be forwarded from the multicast VLAN to the subscriber in a different VLAN. This forwarding behavior selectively allows for traffic to cross between two VLANs.
MVR has been utilized advantageously in metro networks and other environments. However, because all subscriber ports are of a switch are configured as a single VLAN it is not possible to connect multiple VLANs to a single subscriber ports of the layer 2 access switch. New applications are being developed that require a more flexible topology while maintaining the bandwidth saving advantages of MVR for multicast traffic distribution.
The challenges in the field of IP multicast traffic distribution continue to increase with demands for more and better techniques having greater flexibility and adaptability. Therefore, a need has arisen for a new system and method for distributing IP multicast among subscribers.